The invention relates to a process for producing porcelain, in particular for applications in electrical insulation, in which bauxite is used as a starting material. In the text that follows, porcelain of this type is also referred to as bauxite porcelain. The invention also relates to bauxite porcelain and to a ceramic insulator made from the porcelain.
Nowadays, alumina porcelain is customarily used as an industrial ceramic for use in electrical insulation. In this context, the term alumina porcelain is understood as meaning a sintered mixture of alumina, clay, kaolin, feldspar, and, if appropriate, sintering aids and fluxes. In this context, the term alumina denotes a high-purity aluminum oxide and is obtained in a complex manner, using the Bayer process, from the raw material bauxite. Alumina should in particular not be confused with clay, which is usually understood as meaning the weathering product of feldspar-containing rocks that is to be found at secondary deposits. For its part, kaolin is used to refer to the weathering product of feldspar-containing rocks that remains at primary deposits.
Alumina porcelains, which have a high tensile strength, a high bending strength and a high internal compressive strength are used in particular for strength-tested high-voltage insulators. High-strength alumina porcelains have bending strengths, measured on a standardized, glazed bending bar made from the alumina porcelain, of over 170 N/mm2. Depending on the desired bending strength, the amount of alumina to be introduced varies between 27 and 55% by weight, the strength rising as the alumina content increases.
High-strength alumina porcelains are known, for example, from Published, European Patent Application EP 0 189 260 A3, Published, British Patent Application GB 2 056 431 A, U.S. Pat. No. 4,183,760 and European Patent EP 0 522 343 B1.
However, alumina is a relatively expensive raw material thatxe2x80x94as has been statedxe2x80x94has to be obtained in a complex manner from naturally occurring alumina oxide, such as for example bauxite. For this reason, a particularly high-strength alumina porcelain is relatively expensive, which entails drawbacks in particular for mass production for applications in electrical insulation. The price of the alumina represents a considerable burden on the manufacturing and product costs.
It is accordingly an object of the invention to provide a method for producing porcelain, porcelain and a ceramic isolator formed from porcelain which overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, which can be used in particular for highly mechanically loaded components used in electrical insulation. A further object of the invention is to provide porcelain that is less expensive than those used in the prior art while achieving the same mechanical properties. Furthermore, it is an object of the invention to provide a ceramic insulator that is less expensive than conventional insulators known in the prior art while having the same mechanical properties.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for producing porcelain. The process includes mixing calcined bauxite, clay containing more than 5% by weight of first foreign metal oxide inclusions, kaolin containing more than 5% by weight of second foreign metal oxide inclusions, feldspar and magnesium silicate resulting in a mixed compound. The mixed compound is milled and processed into a slurry and the slurry is further processed into a shapeable starting compound. The starting compound is then dried and sintered to produce the porcelain.
According to the invention, the first object is achieved by a process for producing the porcelain, in which calcined bauxite, clay which contains more than 5% by weight of foreign metal oxide inclusions, kaolin which contains more than 5% by weight of foreign metal oxide inclusions, feldspar and magnesium silicate are mixed, milled and processed into a slurry. The slurry is processed further to form a shapeable starting compound, and the starting compound is dried and finally sintered to produce the porcelain. If appropriate, conventional auxiliaries can be added when required.
In other words, in the process described the use of alumina is dispensed with altogether. Instead of alumina, a calcined bauxite is employed, which can be obtained at significantly lower cost than alumina. Calcined bauxite is a raw material that is in the natural state up until the calcining operation. The calcining converts some of the aluminum hydrate contained in the bauxite into aluminum oxide. The use of calcined bauxite allows production costs to be drastically reduced compared to alumina.
The invention is based on the discovery that corundum (xcex1-Al2O3), which is formed from the alumina or the bauxite during firing of the porcelain, is a major factor in ensuring the mechanical strength of the porcelain. Since alumina provides more corundum than calcined bauxite (bauxite still contains impurities), when replacing alumina with calcined bauxite, correspondingly more bauxite has to be employed in order to achieve the same mechanical strength. However, the higher quantity of calcined bauxite required results in that, compared to alumina porcelain, the amount of the plastic components kaolin and clay and of the feldspar that forms the vitreous phase, has to be reduced. However, this in turn entails drastic changes in the mechanical properties of the porcelain.
Extensive tests have shown that the adverse effect of reducing the levels of feldspar and plastic components on the mechanical strength of the porcelain can be compensated for if the plastic components used are a clay and a kaolin, in each case containing more than 5% by weight of the foreign metal oxide inclusions, and magnesium silicate is additionally admixed with the starting materials.
In the case of clays and kaolins, the foreign metal oxides are included in what are known as clay minerals. Examples of clay minerals are sheet silicates, such as kaolinite, illite or montmorillonite.
Surprisingly, it has been found that the foreign metal oxides (impurities) that are included in the clay or kaolin promote the formation of eutectic molten phases during the sintering of the porcelain. The molten phase of the mixture occurs at lower temperatures than the molten phase of the individual components. The sintering temperature of the porcelain can be reduced, which in turn reduces production costs. The particular feature is that the foreign metal oxides that are incorporated in the lattice of the clay minerals have a particularly favorable influence on the formation of the advantageous or aggressive molten phase.
As a result of the aggressive molten phase forming at lower temperatures, it is possible to achieve virtually complete dissolution and conversion of the quartz containing feldspar and kaolin into the vitreous phase. In contrast, in conventional alumina porcelains, there is always a certain proportion of residual quartz. Since inclusions of quartz form imperfections in the microstructure of the porcelain, the porcelain often fractures at locations where quartz particles are included. Therefore, quartz particles per se are undesirable in the porcelain microstructure. Therefore, complete conversion of the harmful quartz into the vitreous phase leads to a considerable improvement in the mechanical strength of the porcelain. The better microstructural properties make the scatter of the strength parameters narrower. The higher damage tolerance also makes the microstructure more stable in terms of long-term performance, which is particularly important for high-voltage insulators.
The use of clays and kaolins which contain more than 5% by weight of foreign metal oxide inclusions therefore results in an aggressive molten phase, which leads to there being scarcely any quartz particles remaining in the finished porcelain. The content of silicon dioxide is virtually exclusively in the form of a vitreous phase. In this way, the bauxite content, which is the main supplier of corundum and therefore of mechanical strength, in the starting substances can be increased considerably without the reduction in the levels of plastic components and feldspar, which is the main source of quartz, having an adverse effect on the mechanical strength of the porcelain. An important side effect is a reduction in the sintering temperature, which additionallyxe2x80x94as stated abovexe2x80x94reduces the production costs and helps to preserve expensive furnace installations and kiln furniture.
Contrary to previous opinions in the specialist field, the invention has found a way in which alumina can be replaced by a significantly less expensive calcined bauxite in order to produce a porcelain of high mechanical strength. The invention demonstrates how the proportion of bauxite, which is the main source of corundum, can be increased without the reduction in the levels of the plastic components and of the feldspar that is required to achieve this having an adverse effect on the microstructure of the porcelain.
The foreign metal oxides iron oxide Fe2O3, magnesium oxide MgO, potassium oxide K2O, sodium oxide Na2O and calcium oxide CaO have proven particularly favorable for the formation of the aggressive molten phase. It is therefore advantageous if the sum of the contents of the foreign metal oxides in the clay or in the kaolin is more than 5% by weight.
In a further advantageous configuration of the invention, a calcined bauxite with an aluminum oxide Al2O3 content of between 80 and 90% by weight is used. In this way, it is possible to introduce a particularly large amount of the main source of strength, namely corundum, into the porcelain with a relatively low bauxite content.
A bauxite of this type is freely obtainable and is sold, for example, by Frank und Schulte, Essen.
To form the aggressive molten phase and to reduce the sintering temperature, it is particularly advantageous if an illitic clay and/or clay that is rich in mixed layer clay minerals is used for production. The term illitic clay is understood as meaning clay that has a high content of the clay mineral illite. The term mixed layer clay mineral is understood as meaning a clay mineral which, compared to kaolinite, does not have an ordered lattice structure, but rather is distinguished by disorders and the lattice structure of which includes large amounts of alkali metal and alkaline-earth metal ions, which act as fluxes. The term illite itself can be used as a trade name for illitic clay. Clay which is rich in mixed layer clay minerals is, for example, the clay which can be obtained as Ball Clay Hymod KC.
As with clay, it is proven particularly advantageous for the formation of an aggressive molten phase if an illite-rich kaolin and/or kaolin that is rich in mixed layer clay minerals is used for production. Illite-rich kaolin is, for example, the kaolin that is mined at the deposits in Oberwinter, Germany. Kaolin that is rich in mixed layer clay minerals is obtained, for example, at the Seilitz, Germany, deposits. The sum of the quantities of iron oxide, magnesium oxide, potassium oxide and sodium oxide in the case of illite is 11.7% by weight, in the case of Ball Clay Hymod KC is 6.1% by weight, in the case of Oberwinter kaolin is 6.4% by weight and in the case of Seilitz kaolin is 5.7% by weight. The detailed composition is given in Table 1, which shows the amounts of foreign metal oxide inclusions referred to for various kaolins and clays.
In a particularly advantageous configuration of the invention, the starting materials used for the production process are, based on the total weight, from 48 to 58% by weight of calcined bauxite, from 10 to 20% by weight of a clay which is rich in mixed layer clay minerals, from 4 to 12% by weight of illitic clay, from 7 to 15% by weight of feldspar, from 0.5 to 3% by weight of magnesium silicate, from 8 to 12% by weight of a kaolin which is rich in mixed layer clay minerals, and from 8 to 16% by weight of illite-rich kaolin. With this composition of the starting materials (in the case of porcelains, one also refers to a xe2x80x9cbatchxe2x80x9d), it is possible to produce bauxite porcelain that satisfies high demands on its long-term aging-free mechanical properties and thermal expansion and is particularly suitable for highly mechanically loaded, large-size insulators that are subject to temperature changes. The mechanical strength can be controlled by the calcined bauxite content. In particular, high-strength bauxite porcelain can be produced using the batch described.
The feldspar used is advantageously a nepheline-syenite. Nepheline-syenite is a feldspathic mineral of empirical formula KNa3(AlSiO4)4 with fluctuating quantities of potassium and sodium. Nepheline-syenite is particularly advantageous with a view to reducing the temperature required for compact sintering.
Furthermore, it is advantageous if steatite is used as magnesium silicate in the production process. Compared to other known magnesium silicates (e.g. talc), steatite has the most favorable properties for production of the bauxite porcelain.
Since calcined bauxite is generally commercially available in a coarse size grain, it is advantageous for the calcined bauxite to be premilled separately before being mixed with the other components. It has also proven advantageous if the calcined bauxite is premilled together with a proportion of the clay. For milling, it is customary to use ball mills, the milling of the calcine bauxite being continued in particular until the grain size of the alumina that is customarily used has been reached.
The corundum content and therefore the mechanical strength of the finished porcelain is decisively influenced by the sintering temperature. With regard to the mechanical properties of the porcelain, it has proven advantageous if sintering is carried out at a temperature of between 1150 and 1300xc2x0 C., in particular between 1190 and 1220xc2x0 C. This is a lower sintering temperature than is customary in the case of conventional alumina porcelains. Furthermore, it has proven advantageous for the invention if the cooling process after the sintering is accelerated by the use of cold air. In this way, rapid cooling of the porcelain is achieved in the sintering kiln. Rapid cooling suppresses transformation of corundum into mullite, and the mullite crystals are kept small. Microstructures of this type are in turn advantageous for the mechanical properties of the porcelain.
According to the invention, the object relating to the porcelain is achieved by porcelain that is obtainable using the production process described above. Bauxite porcelain of this type differs from alumina porcelain in terms of the pore size and distribution. The bauxite porcelain has more pores than alumina porcelain, but the pore size fluctuates less and the pores are more homogeneously distributed. This is clearly advantageous for the fracture behavior and for the strength of the bauxite porcelain. Furthermore, the corundum crystals in the bauxite porcelain have a different form from those in the alumina porcelain. The corundum crystals in the alumina porcelain have an elongate, platelet like form, whereas the corundum crystals in the bauxite porcelain have a substantially round form. Furthermore, the corundum crystals in the bauxite porcelain are virtually twice as large as in the alumina porcelain and in some cases have inclusions, such as for example titanium oxide. These differences can easily be established by comparing microstructures using electron microscope images. Furthermore, the bauxite porcelain is virtually free of residual quartz contents and is therefore superior to the alumina porcelain in terms of long-term performance.
According to the invention, the object relating to the porcelain is achieved by porcelain that contains from 12 to 21% by weight of mullite, from 30 to 46% by weight of corundum, from 40 to 50% by weight of vitreous phase and from 0 to 2% by weight of quartz. Porcelain of this type can be produced by the process described above using calcined bauxite and is therefore a less expensive alternative to conventional alumina porcelain. The low quartz content results in that porcelain of this type acquires very good mechanical properties and a stable long-term performance. In conventional alumina porcelains, the quartz content, at up to 6% by weight, is higher.
A high-strength porcelain with a bending strength, measured on the glazed bending bar in accordance with DIN IEC 60672, of greater than 170 N/mm2 to over 200 N/mm2 is provided by porcelain which advantageously contains from 12 to 15% by weight of mullite, from 38 to 46% by weight of corundum, from 44 to 47% by weight of vitreous phase and from 0 to 1% by weight of quartz.
In a further advantageous configuration of the invention, the grain size of the included quartz particles is from 20 to 40 xcexcm, in which, in a cross section through the porcelain, there are fewer than 10 quartz particles per mm2. Porcelain of this type can also easily be produced using the production process described above. The fact that the porcelain is virtually free of quartz inclusions explains its high mechanical strength and stability in terms of long-term performance. Porcelain of this type satisfies particularly high demands for large-size insulators that are exposed to extremely high mechanical loads.
According to the invention, the object relating to the ceramic insulator is achieved by an insulator, the insulating compound of which contains the porcelain described above.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for producing porcelain, porcelain and ceramic isolator formed from porcelain, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.